1. Technical Field
The present invention relates to an image transfer device and an image forming apparatus incorporating the same, and more particularly, to an image transfer device that transfers a toner image from an image bearing surface to a recording medium, and an electrophotographic image forming apparatus incorporating such an image transfer capability.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is created through a sequential imaging process, including electrostatic charging of a photoconductive surface, exposure of the photoconductive surface to light creating an electrostatic latent image on the photoconductor, and development of the latent image into a visible toner image, followed by transferring the toner image to a recording medium, such as a sheet of paper.
Some image forming apparatuses incorporate an intermediate transfer mechanism, a particular type of image transfer process, which includes a looped intermediate transfer belt with its outer, image bearing surface contacting a drum-shaped photoconductor to form a primary transfer nip therebetween, at which the toner image is transferred from the photoconductive surface to the image bearing surface. The mechanism also includes a pair of opposed transfer members, one being a nip roller outside the belt loop and the other being a backup roller inside the belt loop, disposed opposite each other via the belt to form a secondary transfer nip therebetween, at which the toner image is transferred from the image bearing surface to a recording medium entering the nip in sync with the toner image.
Good imaging quality requires proper conveyance of recording media throughout the imaging process. Thus, for facilitating media conveyance through the transfer nip, the image transfer device may be equipped with a media separator that separates the recording medium from the image bearing surface after exiting the transfer nip.
For example, an image transfer device with an electrically biased media separation capability has been proposed that includes a transfer roller disposed opposite an image bearing member to form a transfer nip therebetween across which an electrostatic transfer bias is applied to transfer a toner image from the image bearing member to the recording medium. A media separator electrode in the form of a roller is disposed downstream from the transfer nip in a direction in which the recording medium is conveyed and removes charge from the recording medium to assist in separating the recording medium from the image bearing surface at the exit of the transfer nip.
In this image transfer device, the transfer roller is connected to a transfer power supply that supplies a transfer bias voltage to the transfer roller to enable electrostatic image transfer across the transfer nip. The media separator roller is connected to a separator power supply different from the transfer power supply that supplies a separator bias voltage to the media separator roller to separate the media from the image bearing surface after exiting the transfer nip.
The media separator roller is positioned extremely close to the transfer nip so as to leave a minimum allowable spacing between the media separator roller and the transfer roller. Such positioning is intended to compensate for a relatively low discharge efficiency of the roller-shaped electrode, which has a relatively large radius of curvature, as compared to that of a thin, needle-shaped electrode, and therefore yields a smaller amount of electric charge induced per unit voltage applied.
According to this method, the transfer power supply, dedicated to the transfer roller, performs constant current control as the transfer roller rotates, whereas the media power supply, dedicated to the media separator roller, performs constant current control during passage of a recording medium through the transfer nip. These power supplies are controlled to have their alternating current components oscillating with identical phase and frequency, so as to prevent interference between the bias voltages applied to the transfer roller and the media separator roller.
Two measures may be adopted to improve performance of electrically biased media separation during image transfer: one is to reduce the gap or spacing between the media separator and the transfer nip, and the other is to increase the amount of separator bias voltage applied to the media separator. Of these, increasing the separator bias voltage is less likely to adversely affect proper media conveyance through the transfer nip, considering that too small a roller-to-nip gap can result in undesired interference between the media separator and the recording medium. However, increasing the voltage applied to the media separator has a limitation in that increased electrical biasing to the media separator can induce a substantial potential difference between the media separator and the transfer roller, leading to an electrical interference and an electric field which eventually causes a leakage current between the media separator and the transfer roller.
Current leakage between the media separator and the transfer roller would cause various adverse effects, such as unwanted adhesion of toner to the transfer roller and the media separator, resulting in soiling or smudges on the back of the recording medium as well as contamination of the surrounding structure inside the imaging equipment. Moreover, increased amounts of leakage current generate significant amounts of ozone through electrical discharge, while disturbing a balance between the current flow from the transfer roller toward the image bearing surface and that from the media separator toward the image bearing surface, resulting in imperfect image transfer and unintended re-transfer of toner from the recording medium to the image bearing surface due to excessive electrical discharge from the media separator.
One possible approach to address the problem is to adjust electrical biases applied to the transfer roller and the media separator from their respective power supplies, such that a change in the separator bias voltage is followed by a corresponding change in the transfer bias voltage. Such adjustment allows for maintaining the transfer bias and the media separator bias equal to each other, which reduces the risk of a large potential difference and concomitant current leakage between the media separator and the transfer roller.
Although theoretically effective, this approach is impractical, however. Adjustment of the transfer bias and separator bias voltages would require complicated feedback control circuitry that initially measures the voltage supplied to the media separator, and then tunes the voltage supplied to the transfer member to be equal to the measured voltage. Moreover, provision of separate, dedicated voltage sources for the transfer roller and the media separator results in a relatively large electrical bias applicator, which adds to overall size and costs of the image forming apparatus incorporating the image transfer device.